Conventionally, a signature method which aims to verify whether or not partial region data (not whole image data) has been altered is proposed in, for example, U.S. Pat. No. 5,898,779. More specifically, in the signature method proposed in U.S. Pat. No. 5,898,779 for a digital image, as shown in FIG. 17, an ROI (region of interest) of the relevant image is first selected (step S131), the hash value (i.e., the message digest) of the selected ROI is calculated (step S132), the calculated hash value is encrypted by a private key to generate a digital signature (step S133), and the generated digital signature is attached to the relevant image (step S134).
Thus, according to such a conventional technique, it is possible to verify whether or not the region data in the original image data has been altered. However, it is difficult in the conventional technique to verify whether or not the relationship between the region data and the original image data is correct. For example, it is difficult to verify whether or not the region data is the partial image data existing in the original image data, and it is also difficult to verify whether or not the partial image data is the region data at the correct location in the original image data. That is, even if the partial image data including the digital signature is falsely added to original image data that is quite different from the authentic original image data, it is impossible to detect such alteration. Moreover, even if the partial image data is falsely replaced by different region data in the authentic original image data, it is impossible to detect such alteration if the digital signature of the relevant different region data exists.